The invention relates to improvements in dryer drum structures for drying a traveling web, and more particularly to a structure which is used for thermally drying a web such as a continuous run of paper which is received at high speed from the press section of a papermaking machine.
More particularly, the invention relates to a particular arrangement and guidance of the web and felts which are threaded over the dryer drums with means for subjecting the web and the zones between felt guide rolls and the drums alternately to a vacuum and a pressure for improved control of the web and improved removal of moisture to enhance the operation of the dryer.
In the dryer section of a thermal dryer such as for a paper machine, the moisture carrying web is received from the press section of the machine and is directed along a serpentine path in wrapped relation with a plurality of steam heated dryer drums. These drums are usually arranged in upper and lower rows with the drums staggered so that the paper web can pass back and forth in a serpentine path and wrap the upper and lower drums with the moisture evaporating from the web as it is heated while in contact with the drum. To aid in the heat transfer from the drum to the web and to support the web in travel between the drums, upper and lower looped felts are provided which wrap the outer surface of the web while on the drum surface.
In relatively high speed papermaking machines, disadvantages have been encountered that the web is not fully controlled in transfer between the upper and lower felts and is sometimes lost at the point of transfer. Further, other disadvantages are encountered in that moisture which is removed by heating is not carried away as fast as possible in the areas between the drums.
It is accordingly an object of the present invention to provide an improved dryer section arrangement which improves the handling and transfer of the web and which also improves the removal of moisture therefrom. This objective generally is desired and sought after because with the increase in speed of papermaking machines, it is necessary for the dryer section to be able to accommodate such increase in speed.
A further object of the invention is to provide a web handling arrangement for a dryer section of a papermaking machine which achieves and accommodates more uniform drying across the width of the sheet and which helps avoid the complications of buildup of air pressure and buildup of moisture in the pockets formed between the felts, the dryer drums, and the guide rolls for the felts.
A feature of the invention is the provision of guide rolls for the felts which are constructed as perforated roll shells with glands within the roll shells that divide the areas into a pressure and vacuum chamber so that the web is alternately subjected to vacuum and pressure as it travels over the roll. A particular feature of the invention is the arrangement whereby the concepts of the inventions can be employed in a dryer drum wherein the web runs in either direction. That is, the same construction can be utilized with one arrangement as a mirror image of the other so that in one arrangement, the web is first subjected to a vaccum and then to pressure, and in the other arrangement, first subjected a pressure and then to a vacuum. This unique arrangement has been discovered to avoid damages in web handling and drying in both arrangements and to substantially facilitate reduction in costs in manufacture and assembly of the dryer arrangement and permit variations to be employed on the site of the papermaker without the requirement of the addition of more expensive parts or expensive and time consuming alteration and reconstruction.